You can just autofill the verification code from your keyboard -- but you can't do it from just any third-party email application. If you want to autofill email verification codes on iOS 17, here ...

A six-digit verification code is sent to your email address as a part of the verification.

Select a sign-in method from the options. Choose between entering a password or a verification code sent to the email address. Select the Enter your password option.

To help make sure that your account is in your control, you must confirm another verification code. For extra security, you must wait for the timer to finish before you can send the code.

Click Send email to have a verification code sent to the email address on file. If the email address is incorrect, click the Email address menu setting under User Info to change it.